In order to obtain the gradient of measured performance, conventional high performance adaptive arrays employ RF or IF correlation. For limiting the correlation, and thus the adaptive processor, to the RF bandwidth of interest and to provide adequately high level RF/IF inputs, there must be effectively incorporated into the system RF and IF stages of a high quality receiver at each antenna input. Since this approach is expensive, it is economically important that, at present, there exist many thousands of unprotected radio links that require antijam protection and that already incorporate expensive high quality radios.
One example of a conventional scheme for minimizing the need for expensive RF/IF correlators is a signal processing system using weight perturbations coupled with the measurement of changes in receiver output performance measure. As is shown in FIG. 1, in such a system the output of each of an array of antennas 1-1 to 1-N is coupled to a respective receiver multiplier 2-1 to 2-N. Each multiplier 2-i multiplies the output x.sub.i (t) by the sum of calculated weight and performance values w.sub.i and p.sub.i (t) supplied to adders 3-1 to 3-N and supplies these products to an adder 4 wherein the products x.sub.1 (t)[w.sub.1 +p.sub.1 (t)] . . . x.sub.N (t)[w.sub.N +p.sub.N (t)] are summed. The output of adder 4 is coupled to a receiver 5 which amplifies and filters the summation signal to the bandwidth of interest. The IF output of receiver 5 is supplied to circuits 6 and 7 in which the total output power Ps+N (signal plus noise) and total signal power Ps are derived. To derive the total power (Ps+N), circuit 6 squares the IF output voltage of receiver 5 and passes the squared voltage through a lowpass filter. To derive the signal power Ps, circuit 7 consists of a matched filter for the described signal followed by a power detector and lowpass filter. The time delay .tau.s through signal power detection circuit 7 is approximately equal to the reciprocal of the bandwidth of the bandpass filter plus the reciprocal of the bandwidth of the lowpass filter. These power values are, in turn, coupled to weight and perturbation value calculator circuit 8 which generates the values w.sub.i and p.sub.i (t) in accordance with a prescribed algorithm. Calculation circuit 8 adjusts the array weights in accordance with whether or not the performance measure is improved as different weight and perturbation values are applied to summing circuits 3-1 to 3-N.
Unfortunately, the above-described conventional scheme suffers from the following drawbacks. First, the performance measure estimate cannot be accurately obtained for a given change in the weight vector until a time delay greater than .tau.s has expired. As a result, the iteration rate of the adaptive system is limited not by the bandpass of receiver 5, but rather by the bandpass of the desired signal estimation filter. Secondly, the estimate of the output desired signal power is in error since some of the receiver output noise is not rejected by the desired signal matched filter; accordingly, the performance measure estimate is biased by some unknown quantity.